In recent years, many thin image display devices as typified by displays, so-called flat panel displays, have been placed on the market, due to such characteristics that they are thinner than cathode-ray tube displays and do not take a large width space. With the advances in production technology, their market price has become reasonable year by year. There has been a growing demand for them, and their production volume has been increasing year by year. Especially, liquid crystal displays have almost reached the mainstream of TVs. Also recently, organic light-emitting display devices such as organic EL displays have been drawing attention as a next-generation image display device, which emits light by itself and thus provides high visibility. Better performances are strongly required of these display devices, such as higher image quality (e.g., advanced contrast and color reproducibility) and lower power consumption.
A color filter is used in these liquid crystal display devices and organic light-emitting display devices. For example, in the case of color liquid crystal displays, colors are produced in the following manner: a back light is used as light source; liquid crystal is electrically driven to control the amount of light; and the light passes through a color filter to produce colors. In liquid crystal TVs, therefore, a color filter is necessary to produce colors, and it plays a major part in influencing display performance. In organic light-emitting display devices, a color image is formed in the same manner as liquid crystal display devices, when a color filter is used in combination with an organic, white light-emitting element.
As a trend in recent years, there has been a demand for power-saving image display devices. To improve backlight use efficiency, there is a strong demand for high-luminance color filters. This is a significant issue, especially for mobile displays (cellular phones, smart phones and tablet PCs).
Although technological advances have provided an improvement in battery capacity, there is still a limit on battery capacity of mobile devices. On the other hand, along with an expansion in display size, power consumption tends to increase. Image display devices comprising a color filter are directly linked to the available battery time of mobile devices or to the charging frequency of batteries in mobile devices, so that they determine the design and performance of mobile devices.
In general, a color filter comprises a transparent substrate, a color layer formed on the substrate and comprising color patterns in three primary colors of red, green and blue, and a light shielding part formed on the transparent substrate to separate the color patterns from each other.
A pigment dispersion method has been widely used for forming such a color layer, in which a pigment with excellent heat resistance and light resistance is used as a colorant. However, it has been getting difficult for color filters formed with a pigment to meet the recent demand for higher luminance.
As a means of achieving higher luminance, photosensitive resin compositions for color filters comprising a dye have been studied. In general, dyes have higher transparency than pigments and enable the production of high-luminance color filters. However, dyes are poor in heat resistance and light resistance and have a problem that the chromaticity is likely to change when heated at high temperature in color filter production process. Also, there is a problem that photosensitive resin compositions comprising a dye are likely to form aggregates in drying process. The aggregates formed in a coating film significantly deteriorate the contrast of the film, making it difficult to use the film as a color layer.
Also, dyes in which the molecules have a salt structure have excellent solubility in polar solvents, that is, solvents as typified by water and alcohols, and they have poor solubility in non-polar solvents or low-polar solvents as typified by propylene glycol monomethyl ether acetate (hereinafter may be simply referred to as PGMEA).
PGMEA or the like has been widely used in pigment-containing photosensitive resin compositions, for the reasons of low risk to human body, excellent heat-drying properties while having low volatility at or around room temperature, etc. Also, a photosensitive resin composition containing large amounts of solvents with very different polarities is problematic in that a washing process is further needed when switched from a conventional PGMEA-containing photosensitive resin composition. In addition, photosensitive resin compositions containing large amounts of polar solvents are not preferable since, due to water absorption, etc., they may cause problems such as unevenness or aggregates upon drying, and there are concerns about production problems. For these reasons, even in the case of dye-containing photosensitive resin compositions, it is demanded to use non-polar solvents or low-polar solvents such as PGMEA and not to contain polar solvents such as water and alcohol.
Moreover, in color filter production process, it is required that the solid content in a photosensitive resin composition has excellent re-dissolubility in solvents. “Re-dissolubility in solvents” means such a property that the solid content of a photosensitive resin composition once dried, is re-dissoluble in solvents. Poor re-dissolubility makes it difficult to remove or clean up a dried product of the photosensitive resin composition attached in color filter production. To improve productivity, there is also a demand for improvement in alkaline developability.
As a dye-containing photosensitive resin composition for color filters, a blue colored composition is disclosed in Patent Literature 1, the composition comprising a blue pigment and a salt-forming compound comprising a xanthene acid dye and a quaternary ammonium salt compound. According to Patent Literature 1, it is explained that there is an improvement in heat resistance, light resistance and solvent resistance as a result of adsorption of the salt-forming xanthene acid dye on the blue pigment. That is, this effect is obtained by having a pigment, and even if a salt-forming compound derived from a dye is used alone or a dispersion of the compound with an acryl resin is used, the solubility of the blue colored composition in PGMEA is not sufficient, and the blue colored composition has a problem of poor optical properties due to color unevenness caused in or aggregates formed in coating film formation.
As a means of increasing the luminance of color filters, a photosensitive resin composition is disclosed in Patent Literature 2, the composition comprising the following as colorants: an organic pigment and a specific, organic solvent-soluble dye comprising xanthene as the basic skeleton. However, in the case of the photosensitive resin composition according to Patent Literature 2, the dye is dissolved in 4-hydroxy-4-methyl-2-pentanone for use, and the heat resistance is obtained by containing the organic pigment.